Alzheimer's disease (AD) is an age-related neurodegenerative disease characterized by progressive cognitive decline and dementia. An important component of AD pathology relates to the accumulation of amyloid plaques in the brain. Recent genome scans have identified ten novel AD susceptibility loci, and several of these loci implicate the immune system in late-onset AD. CD33 is one of these genes; the protein is a transmembrane glycoprotein expressed on the surface of myeloid progenitor cells and mature monocytes and appears to have a constitutive repressor function, subduing monocyte pro- inflammatory cytokine production. Interestingly, our preliminary data shows a strong correlation of increased CD33 expression on monocytes and the CD33 risk alele. Determining the role that altered CD33 expression plays in the basic functions of monocytes, will give important insights into the potential role of this molecule in AD pathology. Using three different collections of human subjects, we will characterize the functional consequences of the CD33 locus in monocytes in (1) younger healthy subjects (2) presymptomatic older subjects, and (3) subjects with AD. We propose a multifaceted approach integrating human genetic and human immunology experiments throughout the life course to understand how the CD33 locus alters the state of activation in monocytes and enhances susceptibility to AD.